Generally, the present invention relates to a magneto-optical apparatus and an optical head that uses a magneto-optical storage medium to which data is written to and/or read from by applying a laser beam and a magnetic field thereto. More particularly, the present invention relates to a magneto-optical apparatus and an optical head that irradiates a beam splitter with the divergent spherical light of a laser beam.
Optical disks have been widely used as external storage mediums for computers. Within the multi-layered structure of an optical disk, and particularly within the structure of a magneto-optical disk, a magnetic recording layer is provided. Since data is written perpendicularly on the magnetic recording layer, the ability to save the recorded contents is good, making it possible to repeatedly read and write new data many times. In a magneto-optical disk, a sub-micron order mark can be written onto the medium by using a laser beam. When compared with floppy disks, magneto-optical disks have much greater storage capacities, and such capacities are being improved even further. For example, when considering a 3.5-inch magneto-optical (MO) disk, it is now possible to store up to 1.35 GB thereon, whereas earlier MO disks had storage capacities of approximately 128 MB. While high storage capacity is important, it is desirable that the magneto-optical disk apparatus used for reading the MO disk is also compact, lightweight, and of a low cost.
FIG. 13 and FIG. 14 are explanatory drawings of prior art devices. FIG. 13 shows a prior art magneto-optical read/write head, and FIG. 14 is a top view of the polarization distribution surface of FIG. 13.
In FIG. 13, the magneto-optical disk medium 96 includes a recording layer that is formed on the top of a substrate, and is made from a magnetic material. This medium 96 makes use of the changes in the heat of the light from a laser and a magnetic field in order to allow information to be written thereon. There are data tracks on this medium for reading and writing the data. Generally, a spiral shaped groove (a tracking groove) is formed on the medium 96. The tracks for reading and writing data are formed on the lands between the grooves.
The light beam of an optical head 90 tracks the spiral track. When writing information, the head 90 makes use of the magnetic field and the changes in the heat of the light in order to write the information. Moreover, when reading the information, the head 90 makes use of the magneto-optical effect to read the information from the light beam reflected back from the disk medium. In order to read or write, the laser beam is focused on the writing surface of the medium. Focus servo control is performed such that the laser beam is maintained in the focused state. In addition, it is also necessary for the light beam to follow the data track. Therefore, track servo control is also used.
In this optical head 90, after the light emitted from the semiconductor laser 91 passes through a beam splitter 93, the light is arranged to become a parallel beam by a collimator lens 94, and then it is focused on the magneto-optical disk 96 by an objective lens 95. On the magneto-optical (MO) disk 96, in the area of the mark, the polarization angle of the light is turned by the Kerr effect. For example, by using the P-polarized light component for detection, the polarization angle is rotated only at the mark area by xcex8K by the Kerr effect in order to generate the S-polarized light component.
The reflected light intensity of the S-component and that of the P-component are changed by the beam splitter surface 93a, which is part of the polarization beam splitter 93. Then, the light is separated into the S and P components by another beam splitter (not shown in the figure), such as a Wollaston prism, and then it is input to the light detection devices. The read signal (called the MO signal) is detected from the difference between the strengths of the two signals.
Part of the light returning from the MO disk 96 is projected onto the beam splitter 93, at which time the focus error signals and the track error signals are detected. In this way, in order to read the signal from the polarized component of the light returned from the MO disk, a polarization separation function is required. When this arrangement of a magneto-optical head is compared with the head for an optical disk, the construction of this arrangement is much more complicated.
In order to simplify the construction of this type of magneto-optical head, the following construction has been proposed in the prior art. Briefly, the divergent spherical wave of a semiconductor laser 91 is projected onto beam splitter surface 93a of a beam splitter 93 (for example, as disclosed in Japanese Patent Application H9-231604). In this proposal, the magneto-optical disk signal detection and the servo detection functions are integrated, making it possible to reduce the number of optical components within the optical head. In addition, it is also possible to reduce the number of places requiring adjustment, as well as to make the optical head more compact and of a lower cost.
However, this prior art configuration has the following problems. In order to project the divergent spherical wave of the semiconductor laser 91 onto the beam splitter surface 93a of the beam splitter 93, the beam is projected onto the beam splitter surface 93a with an inclination. Therefore, as shown in FIG. 13 and FIG. 14, the S-polarized light component is generated by the beam splitter 93. As a result, after the beam has passed the beam splitter 93, the linear polarization component of the light emitted from the collimator lens 94 is slightly rotated.
For signals such as the MO signal, which are detected by determining the difference between the S-polarized light component and the P-polarized light component, it is not possible to eliminate this rotation effect. In other words, as shown in FIG. 14, this rotated component overlaps the bowl-shaped image B of the diffracted light from the groove, so the MO signal component is overlapped when tracking shifts. The tracking shift is not constant, so the amount of overlap changes corresponding to the tracking shift, and MO errors occur. When this happens, it becomes difficult to reproduce the MO signal.
The objective of the present invention is to provide an improved magneto-optical apparatus and an optical head for accurately reproducing and/or reading an MO signal even when the apparatus is made more compact.
A second objective is to provide an improved magneto-optical apparatus and an optical head for accurately reproducing and/or reading an MO signal of a relatively simple design.
Another objective of the present invention is to provide a magneto-optical apparatus and an optical head for accurately reproducing an MO signal even when the apparatus is made more compact, and where the apparatus includes structure for projecting the spherical divergent beam of a semiconductor laser onto the beam splitter surface of a beam splitter.
A further objective of the present invention is to provide a magneto-optical apparatus and an optical head of a configuration that eliminates the signal component that is overlapped with the MO signal when track shifting occurs.
Briefly, the magneto-optical apparatus of the present invention includes a beam splitter that has a beam splitter surface for splitting the path of a light beam into a forward path, which is directed toward a magneto-optical storage medium, and a return path that leads from the medium; a light source for emitting a divergent spherical wave onto the beam splitter surface of the beam splitter in order to emit a spot of light onto the magneto-optical storage medium; an MO detection unit for detecting a magneto-optical signal from a light beam on the return path from the beam splitter surface of the beam splitter; a track error detection unit for detecting a tracking error signal from the light of the return path; and a correction unit for correcting the MO signal by overlapping the tracking error signal with the MO signal.
In this invention, an MO signal, which is overlapped by the tracking error signal, is found that is synchronous with the tracking error signal. By overlapping the tracking error signal with the MO signal, the overlapped error is removed. By doing this, it is possible to easily reproduce the MO signal, even when using a compact optical head as long as that head emits a divergent spherical wave onto the beam splitter surface of a beam splitter. More particularly, it is possible to accurately reproduce the MO signal even when the P-polarized light component and the S-polarized light component are difficult to separate due to increased density.
In another embodiment of the magneto-optical apparatus of this invention, the beam splitter surface of the beam splitter is curved in one direction. Therefore, it is possible to reduce its dependence on the angle of incidence of the laser beam, thereby preventing a reduction of the amount of light. In addition, the reproduction of the MO signal becomes more accurate.
In yet another embodiment of the magneto-optical apparatus of this invention, the beam splitter surface of the beam splitter passes through the divergent spherical wave and is projected along the forward light path with P-polarization. In this other embodiment of the magneto-optical apparatus of this invention, the beam splitter surface of the beam splitter reflects the divergent spherical wave, and is projected along the forward light path with S-polarization.
The optical head of the present invention includes a beam splitter that has a beam splitter surface for splitting the path of the light beam traveling both to and from a magneto-optical storage medium, a light source for emitting a divergent spherical wave onto the beam splitter surface of the beam splitter that emits a spot of light onto the magneto-optical storage medium, an MO detection unit for detecting the magneto-optical signal from a light beam on the return path from the beam splitter surface of the beam splitter, and a xc2xd xcex wavelength plate that is located between the magneto-optical storage medium and the beam splitter, and where the direction of linear polarization of the light emitted from the beam splitter with respect to the magneto-optical storage medium is set such that it is orthogonal to a groove of the magneto-optical storage medium.
In this embodiment of the present invention, in order to obtain the tracking signal in the return light path, the direction of linear polarization is set to be orthogonal to the groove so that the bowl-shaped image does not overlap with the rotated component of polarization, which makes it is possible to prevent the MO signal component from being overlapped by the tracking error signal. In addition, it can be realized by the construction of the present optical head.